Permanent magnet motor with field weakening

ABSTRACT

A permanent-magnet electrical machine is disclosed in which the rotor or stator have at least one movable iron segment. A magnetic field of the electric machine is weakened when the movable iron segment is moved a position away from the rotor or stator, respectively. When the movable iron segment is in a first position, such as in contact with the rotor or stator, the field strength is high. When the movable iron segment is in a second position in which the movable iron segment is displaced away from the rotor or stator, the field strength is low. The ability to weaken the field strength causes the constant-power, speed ratio to be increased and thereby increases the utility of the electric machine for applications in which a wide speed range is desired. The electric machine may be used as both a permanent-magnet motor and generator.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.13/315,872, filed Dec. 9, 2011 which claims the benefit of U.S.provisional Application No. 61/421,952 filed Dec. 10, 2010, thedisclosures of which are incorporated in their entirety by referenceherein.

TECHNICAL FIELD

The present disclosure relates to magnetic field weakening in apermanent magnet motor.

BACKGROUND

There is a need for efficient electrical machines that have high torquecapability over a large speed range and the ability to control machinespeed, in particular for electrical drives for vehicles, such aselectric or hybrid vehicles, or other electric generation applicationswhich require high torque at zero and low speed.

For purposes of providing traction power, such as in electric vehicles,it is desirable to have an electric motor with a high constant powerspeed ratio (CPSR). Referring to FIG. 1, torque and power as a functionof speed is shown for an electric motor. At low speed, high torque isavailable with such torque assisting with launch. As N_(min) is reached,the motor's maximum power is accessed and no more power is available asspeed is further increased. Recalling that P=2*Π*T*N; as power, P, isconstant, as speed, N, is increased, torque, T, reduces. CPSR is themaximum speed at which rated power can be delivered (N_(max)) divided bythe lowest speed at which maximum power is available (N_(min)). N_(min)is also the highest speed at which rated maximum torque can bedelivered. The maximum speed (N_(max)) is limited primarily by a limiton back EMF voltage, and also by damage to the rotor or other inherentlimitations of the motor. For example shown in FIG. 1, the CPSR is afactor of two.

It is desirable to have a CPSR of four or more for automotiveapplications. Although it is possible to achieve that with inductionmotors, motors with field coils, or switched reluctance motortechnologies, permanent magnet motors are preferred due to their higherpower density and higher efficiency. Permanent magnet (PM) motors,however, do not inherently have CPSRs in such a high range. Asignificant amount of effort is being expended in determiningcost-effective, lightweight, and efficient solutions to address thelimited CPSR of PM motors.

One alternative is to provide a transmission between the electric motorand the final drive. However, transmissions are heavy, costly, and mustbe controlled, either by the operator or by a controller. Anotheralternative is to electrically adjust the field strength of the electricmotor if it has electrically excited field windings. This approach isnot available to motors with permanent magnet fields.

Another approach to is to weaken the magnetic field, thus increasing themotor speed for a given back EMF or applied voltage. For any givenmotor, torque produced is proportional to current multiplied by magneticfield strength, while RPM is proportional to voltage/field strength. Sofor a given power (voltage*current) in, a motor makes a certain amountof mechanical power, (T*N). If the magnetic field is weaker, the motormakes the same power but at higher speed and lower torque.

In an electric motor, there is an air gap between the rotor and thestator. The motor is usually designed to have as small an air gap aspractical. The field strength can be weakened, however, by increasingthat air gap. Such a system has been employed in axial flux motors, inwhich the rotor and the stator are substantially disk shaped. Thedisplacement between the two disks can be increased to reduce the fieldstrength. In a radial flux motor, the rotor may be centrally locatedwith the stator arranged outside the rotor circumferentially displacedfrom the rotor. If the rotor, for example, is displaced along the axisof rotation, the effective field strength of the radial flux motor isreduced. The mechanisms that adjust the relative positions of the rotorand stator are relatively expensive and yield a more cumbersome motor.In alternatives in which a portion of the windings are switched off orthe relative positions of the rotor and stator are adjusted, anelectronic controller commands the adjustments based on input signals.Such controllers can be costly.

SUMMARY

According to embodiments of the present disclosure, the field strengthof the motor is altered by adjusting the reluctance of the back iron ofat least one of the rotor and the stator. By providing the back ironwith both a thin, fixed back iron portion, or in some embodiments noneat all, and a movable back iron portion, adjustments in the fieldstrength are possible. When the movable back iron portion is in contactwith the fixed back iron, the two act as one larger back iron. When themovable back iron portion is displaced from the fixed back iron, thefixed back iron is substantially the full extent of the back iron.Almost all the magnetic flux has to pass through this thin fixed backiron section, so the fixed back iron is “saturated” or its “magneticresistance” or reluctance goes up, thereby reducing the field strength.

In embodiments in which movable back iron segments are applied to therotor, and the rotor is external around a central stator, the actuationof the back iron segments between the first position (in contact withthe fixed back iron) and the second position (separated from the fixedback iron) can be effected by centrifugal force. There is a smallmagnetic force causing the fixed and movable back irons to remain incontact. However, as the speed of the rotor increases, the centrifugalforce can overcome this weak attraction causing the movable back ironsegments to move away from the fixed back iron. In such an embodiment, atray or other retainer can be provided to catch the movable back ironsegments as they move away from the fixed back iron. As the rotor speeddecreases, the movable back iron segments may be drawn back to the fixedback iron due to the magnetic force between the two. In otherembodiments, the movable back iron segments are tethered to the fixedback iron by springs or by tethering linkages that are spring loaded toprovide a biasing force toward the fixed back iron. In some embodiments,the movable back iron segments move at different speeds so that asmoother transition in field strength as a function of rotational speedcan be provided. The back iron segments react at different speeds due todiffering weights by using differing density materials, a range ofthicknesses or footprint sizes. In embodiments in which movable backiron segments are biased via a spring, the spring tension can beadjusted to provide the desired response. Mechanical, electrical,pneumatic or hydraulic actuators can also be used to move the rotor backiron segments.

In FIG. 2, a motor in which there are three ranges of field strength isshown. The CSPR is two times, just like that shown in FIG. 1. Thus, forthe first range of field strength, there is a Nmin1 and a Nmax1 that arein the ratio of 1:2. There is also a second range of field strength thatyields a N_(min2) and a N_(max2) also in the ratio of 1:2. If N_(min2)were equal to N_(max1) and N_(min3) equal to N_(max2), the resultingCSPR is eight. As it might be desirable to have N_(min2) be a littleless than N_(max1), the resulting CSPR would be somewhat less thaneight.

In other embodiments, the movable back iron segments are applied to therotor using an actuator to move them. In passive control made possibleby centrifugal force acting on the movable back iron segments on therotor, rotor speed is the only factor by which the movable back ironsegments are adjusted. By actively controlling the actuator, the demandfor torque by the operator, temperatures in the motor or a battery packcoupled to the motor, state of charge of the battery, or other factorscould be inputs to the electronic control unit that commands control ofthe actuator. A plurality of back iron segments as well as a pluralityof actuators can be employed to provide a series of steps in fieldstrength.

In yet another embodiment, the field strength of the motor can beweakened by affecting the reluctance of the stator ring. This can beaccomplished by having a fixed stator ring and one or more movablestator ring segments. Because the stator is not rotating, an actuator isused to cause the movable stator ring segments to separate from thefixed stator ring.

Also disclosed is a method to operate an electric motor in which thestator has a fixed back iron and movable back iron segments. The movableback iron segments are moved by an actuator between a first position inwhich the movable back iron segments are in contact with the fixed backiron and a second position in which the movable back iron segments aredisplaced from the fixed back iron. An electronic control unit commandsthe actuator to move the movable back iron segments based on one or moreof motor speed, demand for motor torque, motor temperatures, and stateof charge of a battery supplying electricity to the motor. In oneembodiment, a desired field strength is determined based at least on thespeed of the motor. An electronic control unit (ECU) commands anactuator coupled to the movable back iron segments to provide thedesired field strength in a system with a continuously variable fieldstrength and to approximately provide the desired field strength in asystem in which the field strength is stepwise variable.

In some embodiments, the desired field strength is further based on theoperating mode. For example, the state of charge of the battery affectsthe optimum field strength, i.e., that which provides good efficiency.Also, battery regeneration or charging requires a field strength (highervoltage condition) than battery discharging. Thus, such informationprovided to the ECU is used to select the desired field strengthsuitable for the operating mode.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 are graphs of torque and power provided by a motor as afunction of motor speed;

FIG. 3 is an illustration of an electric motor powered scooter;

FIG. 4 is a cross section of a portion of an electric motor;

FIG. 5 is a portion of a cross section of a portion of a radial fluxelectric motor incorporated into a wheel;

FIGS. 6 and 7 are end views of the rotor and stator of the axial fluxmotor with movable back iron segments in contacting and non-contactingpositions, respectively;

FIGS. 8 and 9 show a cross section of a rotor for an axial flux motorwith a movable back iron segment tethered to the periphery of the rotor;

FIG. 10 shows a cross section of a rotor having movable back ironsegments of varying thicknesses;

FIGS. 11-13 illustrate stators with back iron segments movable byactuating systems;

FIG. 14 illustrates an equivalent electrical circuit that can be used toanalyze the magnetic circuit of the motor, with back iron represented byresistors, movable back iron is shown in parallel with the stator androtor resistors, with switches to represent them being disconnected whenmoved away. When the back iron is in contact, the switch is closed andthe two resistors are in parallel, so their total resistance is lower.

FIG. 15 shows a cross section of an internal rotor motor with a movableback iron segment illustrating a state in which the movable back ironsegment is contact with the fixed back iron segment;

FIG. 16 shows a cross section of the internal rotor motor of FIG. 15illustrating a state in which the movable back iron segment is separatefrom the fixed back iron segment;

FIG. 17 is a graph of torque vs. rpm for three levels of field strength;

FIG. 18 shows a family of curves at constant power on a voltage-currentgraph; and

FIG. 19 shows optimum field strength as a function of rpm for threeoperating modes: normal drive, low battery, and battery regeneration.

DETAILED DESCRIPTION

While the best mode has been described in detail with respect toparticular embodiments, those familiar with the art will recognizevarious alternative designs and embodiments within the scope of thefollowing claims. While various embodiments may have been described asproviding advantages or being preferred over other embodiments withrespect to one or more desired characteristics, as one skilled in theart is aware, one or more characteristics may be compromised to achievedesired system attributes, which depend on the specific application andimplementation. These attributes include, but are not limited to: cost,strength, durability, life cycle cost, marketability, appearance,packaging, size, serviceability, weight, manufacturability, ease ofassembly, etc. The embodiments described herein that are characterizedas less desirable than other embodiments or prior art implementationswith respect to one or more characteristics are not outside the scope ofthe disclosure and may be desirable for particular applications. Asrequired, detailed embodiments of the present invention are disclosedherein; however, it is to be understood that the disclosed embodimentsare merely examples of the invention that may be embodied in various andalternative forms. The figures are not necessarily to scale; somefeatures may be exaggerated or minimized to show details of particularcomponents. Therefore, specific structural and functional detailsdisclosed herein are not to be interpreted as limiting, but merely as arepresentative basis for teaching one skilled in the art to variouslyemploy the present invention.

As those of ordinary skill in the art will understand, various featuresof the embodiments illustrated and described with reference to any oneof the Figures may be combined with features illustrated in one or moreother Figures to produce alternative embodiments that are not explicitlyillustrated or described. The combinations of features illustratedprovide representative embodiments for typical applications. However,various combinations and modifications of the features consistent withthe teachings of the present disclosure may be desired for particularapplications or implementations. Those of ordinary skill in the art mayrecognize similar applications or implementations whether or notexplicitly described or illustrated.

A motor scooter 10 is illustrated in FIG. 3. The motor scooter 10 has aframe 12 to which an axle 14 is coupled. The axle 14 is coupled throughthe center of a wheel 16, the wheel 16 being rotatable with the axle 14.The wheel has a rim 22 onto which a tire 24 is mounted. As described inmore detail in FIG. 5, a stator may be coupled to the axle 14 and arotor may be coupled to the rim 22. The stator and rotor are elements ofan electric motor powered by an onboard battery (not separately visible)behind a cover 26. The motor scooter 10 has operator controls 28 and 30through which the operator can indicate a demand for power and/orbraking Operator controls 28 and 30 may be coupled electronically to anelectronic control unit (not shown in FIG. 3).

A form of electric motor has a cylindrical rotor surrounded by acylindrical shell stator, commonly called an internal-rotor motor. Therotor and stator are separated by a small air gap. An internal rotormotor may also be a radial-flux motor because the magnetic flux passesin the radial direction across the air gap between the rotor and stator.Another form of electric motor is an external-rotor radial-flux motor,which has a fixed internal stator surrounded by a cylindrical shellrotor. Another type of motor may have a disc-shaped rotor and stator,and is called an axial-flux motor because the flux passes in the axialdirection between the rotor and stator.

While the axial-flux motors and radial-flux motors discussed, thepresent disclosure relates to electrical machines including: dedicatedmotors, dedicated generators, and those that switch between operating asa motor and a generator. The present disclosure applies to all suchelectrical machines. In FIG. 3, an electric motor scooter is shown.However, the present disclosure relates to all motor vehicles:automobiles, electric bikes, etc. and even more broadly to all permanentmagnet electrical machines.

A cross-sectional detail of an electric motor 50 according to anembodiment of this disclosure is shown in FIG. 4. The motor 50 has arotor 52 and a stator 54 separated by an air gap 56. For convenience ofillustration, the rotor 52 and the stator 54 are shown as linearelements. However, it is more common for the rotor 52 to rotate withrespect to the stator 54. In one configuration, the axis about which therotor spins is 60 and the rotor 52 and stator 54 are curved in thedirection of arrows 62. In alternative configuration known asexternal-rotor, or sometimes referred to an inside-out motor, the axisabout which the rotor spins is 64 and the rotor 52 and stator 54 arecurved in the direction of arrows 66.

Continuing to refer to FIG. 4, rotor 52 has a fixed back iron 70 and aplurality of permanent magnets 72 affixed to a surface 71 of fixed backiron 70 that is proximate stator 54. Adjacent permanent magnets 72 havethe opposite polarity, i.e., the north pole of the magnet is proximatethe south pole of the adjacent magnets. The rotor 52 also has aplurality of movable back iron segments 74 positioned adjacent to a rearsurface 75 of the fixed back iron 70 distal from the stator 54. As willbe described in more detail below, the moveable back iron segments 74may be movably attached to the fixed back iron 70. In anotherembodiment, the electric motor 50 have a relatively thin fixed back iron72 or no fix back iron at all, in which case, the moveable back ironsegments 74 are positioned adjacent the permanent magnets 72 or anon-magnetic support structure.

As further illustrated in FIG. 4, the stator 54 has a plurality of slotsor channels 78. The slots or channels 78 are wider at a distal end 79 asthe slots 78 extend away from air gap 56 and the slots 78 are narrowerproximate the air gap 56. T-shaped posts 80 are formed between the slots78. Many wraps of a wire 84 are wound around the T-shaped posts 80 suchthat the wire 84 extends outwardly from a stator back iron 82 throughthe slots 78. Multiple wraps of wire windings 84 are shown in crosssection within slots 78. Also shown in FIG. 4 are magnetic flux lines88.

In FIG. 5, an electric motor is illustrated according to FIG. 4 isintegrated into a wheel 90. A hub 92 has spokes 93 supporting the stator94 and rotates about an axis or an axle 96. An air gap 98 separatesstator 94 from rotor 100. The rotor 100 has a fixed back iron 102 aswell as movable back iron segments 104. Movable back iron segments 104are shown in a first position in which they are in contact with fixedback iron 102. Movable back iron segments 104 are held onto fixed backiron 102 by magnetic attraction, in one embodiment. Alternatively,movable back iron segments 104 are biased toward the fixed back iron 102by a spring loaded tether or a spring. When the rotor 100 rotates, themovable back iron segments 104 separate or move a distance from thefixed back iron 102 when the centrifugal force overcomes the magnetic orspring force. A tray 106 is provided to contain movable back ironsegments 104 when separated from fixed back iron 102. An outer surface108 of the tray 106 forms the rim for mounting a tire 110.

In FIGS. 6 and 7, a rotor 120 of an axial-flux motor configuration isshown. As illustrated in FIGS. 6 and 7, the rotor has a fixed back iron122 and movable back iron segments 124 coupled to the fixed back iron122 with a locating device to contain the movable back iron segments 124from moving from the fixed back iron 122 more than a predeterminedmaximum distance. The locating device may include at least one tether126. In one embodiment, the tether 126 may be biased or spring-loaded.In other alternatives, movable back iron segments 124 may be tethered bytethers 126 without being biased so that the movable back iron segments124 are allowed to float radially outward with respect to back ironsegments 142 with centrifugal force. The tethers 126 prevent themoveable back iron segments 124 from moving outward more than a smalldistance.

As illustrated in FIGS. 6 and 7, the tethers 126 are coupled to movableback iron segment 124 at joint 126a and coupled to fixed back iron 122at joint 126b. A side view of rotor 120 separated by an air gap 128 fromstator 130 are shown in FIG. 7. The movable back iron segments 124 arein contact with the fixed back iron 122 in FIG. 7. In FIG. 8, however,rotor 120 is spinning such that the movable back iron segments 124separate a distance from back iron 122 due to centrifugal force therebyforming an air gap therebetween. Stator 130 includes coils of wire whichare not shown FIGS. 6 and 7.

FIG. 8 illustrates a cross-section view of a rotor 138 for an axial-fluxelectric motor having a fixed back iron 140 and movable back ironsegments 142 coupled to together by a locating device including a spring144 and guide pin 145. The spring 144 may bias the moveable back ironsegments 142 away from the fixed back iron 140, or may also provide abiasing force to a return the movable back iron segments 142 backtowards the fixed back iron 140. Illustrated in FIG. 8 is the situationwhen rotor 138 is stationary or rotating at a speed at which centrifugalforce acting on movable back iron segment 142 is less than the springtension acting on back iron segment 142.

FIG. 9 is an illustration of rotor 138 rotating above the thresholdspeed so that back iron segment 142 moves radially away from fixed backiron 140 due to centrifugal force. Fixed back iron 140 and movable backiron segment 142 are slightly angled so that as movable back ironsegment 142 moves outward radially, a small axial gap between the fixedback iron 140 and movable back iron segment 142 develops. So that thegap forms, the guide pin 145 coupled to fixed back iron 140 slides in asleeve within movable back iron segment 142.

In FIG. 10, a portion of a rotor 148 in a radial-flux external-rotormachine is shown with a fixed back iron 150 and groups of movable backiron segments 152, 154 and 156. The three groups of movable back ironsegments 152, 154 and 156 may have different thicknesses or weights suchthat one group of moveable back iron segments separate from fixed backiron 150 at a lower speed than the other movable back iron segments. Bychanging the reluctance of the back iron in steps, the magnetic fieldchanges more gradually as the speed moves through the ranges oftransition. In other alternatives, the various movable back iron segmentgroups are made of materials of differing density so that the movableback iron segments have different weight. In another alternative, theback iron segments are tethered either by spring-loaded tethers orsprings. The spring tension of the different groups is different toprovide the desired response, i.e., separation of the groups indifferent speed ranges.

Alternatives for altering the reluctance of the back iron of the rotorare described above in which the movable back iron segments are actedupon by centrifugal force, thus moving based on rotor rotational speed.Alternatively, reluctance of the stator ring can be adjusted to affectthe field strength. However, because the stator does not rotate, nocentrifugal force acts upon the movable stator segments and thus anactuator is used to provide the movement of the stator segments.

In FIG. 11, a stator 160 in an external-rotor radial-flux machine isshown with a fixed stator ring 162. Movable stator segments 164 areshown in contact with fixed stator ring 162. Posts 166 are provided withthreads 165. Left hand threads 165 are provided on one end, right handthreads 165 are provided on the opposite end, and gear teeth 168 thatcan be engaged by small electric motors 170, such as stepper motors. Byrotating posts 166 in one direction, movable back iron segments 164 areseparated from fixed stator ring 162. By rotating posts 166 in anotherdirection, movable stator segments 164 are returned to the positionshown in FIG. 11 in which they are in contact with fixed stator ring162. Coils of wire in the stator 160 and the permanent magnet rotorsurrounding the stator 160 are not shown in FIG. 11.

An electronic control unit (ECU) 172 commands operation to motors 170.ECU 172 receives inputs 174 from various sensors which provide signalfrom which one or more of motor speed, current flowing in the motorwindings, voltage across the motor, speed demanded by an operator of amotor vehicle, torque demanded by the operator, braking force demandedby the operator, system temperatures, state of charge of battery 178,geographical position, etc. may be determined. ECU 172 may also commandvarious functions, i.e., provide control outputs 176, based on theinputs 174.

In FIG. 12, a stator of an axial-flux machine has a fixed stator ring180 having a shallow ramp 182 is shown in an edge view. Fixed statorring 180 is disk shaped. In contact with fixed stator ring 180 is amovable stator ring 184 that couples to a stepper motor 186 or otheractuator. Movable stator ring 184 has an inward ramp 188 that coupleswith ramp 182. When movable stator ring 184 is rotated in the directionindicated by the arrow by action of motor 186, movable stator ring 184is caused to separate from fixed stator ring 180 by ramp 188 riding upramp 182. A series of such ramps are provided on the periphery toproperly support the movable stator ring.

The magnetic system can be illustrated and analyzed through a simplifiedequivalent electrical circuit model as shown in FIG. 14 in which rotor200 has fixed back iron 202 and movable back iron segments 204 andmagnetic reluctance is modeled as resistance. Stator 206 has fixed backiron or ring 208 and movable back iron segment or ring 210. The motormay not have both movable back iron segments 204 and 210, however, bothare included in FIG. 14 for illustration of the model. Permanent magnets212 are provided on the surface of fixed back iron 202. Stator 206 has aseries of slots or channels 214 into which windings are wrapped. An airgap 216 is maintained between rotor 200 and stator 206. Fixed back iron202 is modeled as a resistor 220 of resistance R_(Rf). As movable backiron segments 204 are not in contact with fixed back iron 202, theycontribute negligibly to the flow of current. However, when elements 202and 204 are in contact, resistor 220 has the resistance of the fixedback iron in parallel with the resistance of the movable back iron tomake a lower total resistance. Similarly, stator 206 is modeled as aresistor of resistance R_(Sf) when the fixed stator ring 208 and movablestator ring 210 are separated. Adjacent magnet 212 is voltage sources,V₁ and V₂. Air gap 216 is modeled as resistances R_(A1) and R_(A2).Movable back iron segments 204 can be modeled as resistances R_(Rm)which are in parallel with the resistance of the fixed back iron,R_(Rf). However, as shown in FIG. 14, movable back iron segments 204 arenot in contact with fixed back iron 202. To model such a configuration,an open switch, Sw_(R), is shown in FIG. 14. When the movable back ironsegments 204 are in contact with fixed back iron 202, switch Sw_(R) isclosed. Analogously, movable stator back 210 is modeled as a resistor,R_(Sm), in parallel with resistor R_(Sf) modeling fixed stator ring 208.However, as shown, movable stator ring 210 is not in contact with thefixed stator ring 208 and thus switch S_(wS) is shown in FIG. 14 asbeing open.

In the examples illustrated in the Figures, permanent magnets are shownaffixed to the rotor. In some applications, however, the permanentmagnets are cast into openings in the rotor, such as by sinteredmaterials, to thereby capture the magnets to prevent them from becomingdetached from the rotor and to change the magnetic properties of themagnet and rotor assembly. The present disclosure is applicable to theseinternal permanent magnet rotor configurations as well.

In FIG. 15, a cross section of a portion of an internal rotorradial-flux electric motor 250 illustrates an embodiment in which amovable back iron segment is actuatable. An axle 252 is supported onball bearing 254 in a bearing assembly 256. An actuator rod 258 having aflange portion at one end is mounted within axle 252. In FIG. 15, acable 260 is coupled to actuator rod 258. By applying or releasingtension on cable 260, rod 258 is caused to move with respect to axle252. Motor 250 has a stator with windings 262. The rotor includes afixed back iron 264 onto which magnets 266 are affixed. Fixed back iron264 is coupled to a support 265 of the rotor that rotates on bearingassemblies 256. Movable back iron segments 270 are provided, with onlyone such movable back iron segment 270 shown in FIG. 15. A bell crank272 is coupled to fixed back iron 264 via a pivot joint 274 and is fixedto movable back iron segment 270 via a pivot joint 276. Bell crank 272contacts the flange end of actuator rod 258. In FIG. 15, movable backiron segment 270 abuts fixed back iron 264.

Referring now to FIG. 16, actuator rod has been displaced to the leftwith respect to axle 252. Due to the flange end of actuator rod 258moving to the left, bell crank 272 rotates clockwise around pivot joint274 thereby pulling movable back iron segment 270 downward. An air gap278 now exists between fixed back iron 264 and movable back iron segment270. Air gap 278 weakens the field strength. In the movable back ironsegments 306 being in position with an air gap 308. One bell crank 272may be provided to actuate each movable back iron segment 270. Bellcranks 272 may be coupled to an axle.

The discussion of an internal rotor motor in relation to FIGS. 15-16 isdirected to one example of a motor in which the movable back ironsegments can be actuated under external control. In the example shown, acable is shown. However, an electrical actuation via a linear actuator,hydraulic actuation, or a host of other actuation schemes may bealternatively used. The actuator may be under operator control or becontrolled by an electronic control unit. For example, the electroniccontrol unit may be provided signals concerning vehicle and motorparameters and command the movable back iron segments to moveaccordingly. Also, the particular mechanical configuration having a bellcrank that is moved via an actuator rod is not intended to be limiting.Many other suitable configurations could be used instead.

In FIG. 17, a graph of torque as a function of motor RPM is shownaccording to an embodiment of the disclosure. The solid curve 320 showsa situation for a motor with a fixed field strength in which a largespeed range is desired. The constant torque region occurs over a widerange of speed. However, there are two problems with such operation. Themaximum torque is limited. Furthermore, the operation is not nearly asefficient as desirable across the speed range. Curve 322 shows asituation in which the field strength is greater. The maximum torque isgreater, but the speed is limited to that shown as 324. A motor witheven a greater field strength has characteristics of curve 326, with agreater torque at the lower motor speeds, but a very limited range inspeed. The limits in speed are due to the back EMF in the motor becomingexcessive. By providing three ranges of field strength, the motor'sdynamic range improves tremendously and with good efficiency. Thus, ifan increase in motor speed is requested from a starting at point A inwhich the motor speed is low and the desired torque is high, the highfield strength can accommodate accessing point B, which also provides ahigh torque. Beyond point B, the torque must drop, but the power isconstant, i.e., along B to C. However, very little further increase inmotor speed is possible unless the field strength is weakened, such asillustrated by curve 322. By doing so, points D, E, and F areaccessible, with D to E being a constant torque/increasing power, andpoints E to F being at a constant power/decreasing torque. Likewise, afurther decrease in field strength allows accessing points G and H. Byproviding three levels of field strength, a high torque can be providedat low speed along with a wide speed range and near a peak efficiencycondition across the range of speed.

It is desirable to operate the motor at the minimum-current/high-voltagepoint to generate the demanded power because the resistive losses arerelated to current squared and thus losses are minimized at low current.Consequently, the motor's efficiency is improved and undesirable heatingof the motor is reduced. An additional benefit is that the powerelectronics associated with the motor does not step down the batteryvoltage as much, so the electronics can be simpler and more efficient.By selecting field strength data in the desired range, a family ofcurves for a range of powers and speeds can be generated, as shown inFIG. 18. Thus, to operate the motor at an efficient condition at aparticular speed and power, the field strength is varied per therelationship shown in FIG. 18, i.e., dependent on motor rotationalspeed.

Referring now to FIG. 18, a control strategy is illustrated graphically.FIG. 18 graph of voltage as a function of current is shown for a rangeof power levels. Curve 332 may represent, for example, 25 W; curve 334represents 50 W; and curve 336 may represent 100 W. The voltage islimited such that operation above line 338 is not possible. For example,if a battery coupled to the motor is a 12 V battery, the limit of line338 is 12 V. It is more efficient to operate at the lowest possiblecurrents. Thus, the desired operating range is shown as a highefficiency area 340 in FIG. 18. As the motor speed changes, the controlsystem acts to change the field strength to maintain operation in thishigh efficiency area 340.

In FIG. 19, the optimum field strength as a function of motor speed isshown for normal operating mode, e.g., at 72 V, curve 350. When thebattery is getting low, the battery voltage drops and the optimum fieldstrength drops, as shown in curve 352. In a battery regeneration mode,e.g., during regenerative braking in an electric vehicle, the voltage ishigher than the battery voltage and the optimum field strength ishigher, shown as curve 354. In embodiments in which the field strengthis continuously variable, the field strength is selected based on bothmotor RPM and the operating mode, normal, low battery, regeneration,etc. In embodiments in which the field strength is stepwise variable,the field strength step is selected to be as close as possible to theoptimum field strength as a function of motor RPM and operating mode.

By maintaining the field strength within the band labeled as desired,the losses are minimized. This can be accomplished by continuouslyvarying the field strength, such as by a continuous actuator moving oneor more movable back iron segments away from the fixed back iron or insteps by actuating as many of the segments as indicated to provide thedesired field strength, i.e., that which allows current to be at or nearthe minimum. The description above also applies to an electric machineoperated as a generator.

While various embodiments are described above, it is not intended thatthese embodiments describe all possible forms of the invention. Rather,the words used in the specification are words of description rather thanlimitation, and it is understood that various changes may be madewithout departing from the spirit and scope of the invention.Additionally, the features of various implementing embodiments may becombined to form further embodiments of the invention.

1. An electric machine, comprising: a stator; and a permanent magnetrotor having at least one movable back iron wherein a magnetic field ofthe electric machine is weakened when at least one of the at least oneback iron is moved away from the rotor.
 2. The electric machine of claim1, further comprising: a controller configured to command at least oneof the at least one movable back iron to move based on an operatingcondition of the electric machine.
 3. The electric machine of claim 2wherein the operating condition comprises a rotational speed of theelectric machine.
 4. The electric machine of claim 2 wherein theoperating condition comprises a requested power level output of theelectric machine.
 5. The electric machine of claim 2, furthercomprising: an actuator in communication with the controller and coupledto the at least one movable back iron, wherein the controller commandsthe actuator to move at least one of the at least one movable back ironbased on the operating condition.
 6. The electric machine of claim 3wherein the at least one movable back iron comprises a plurality ofmovable back irons, wherein one of the plurality of movable back ironsis moved away from the rotor at a different rotational speed than atleast one other of the plurality of movable back irons.
 7. An electricmachine, comprising: a permanent magnet rotor; and a stator having atleast one movable ring segment, wherein a magnetic field of the electricmachine is weakened when at least one of the at least one ring segmentis moved away from the stator.
 8. The electric machine of claim 7,further comprising: a controller configured to command at least one ofthe at least one movable ring segment to move based on an input receivedby the controller.
 9. The electric machine of claim 8 wherein the inputcomprises a rotational speed of the electric machine.
 10. The electricmachine of claim 8 wherein the input comprises a requested power leveloutput of the electric machine.
 11. The electric machine of claim 8,further comprising: an actuator in communication with the controller andcoupled to the at least one movable ring segment, wherein the controllercommands the actuator to move at least one of the at least one movablering segment based on the input.
 12. The electric machine of claim 7wherein the movable ring segment is adapted to move in a radialdirection.
 13. The electric machine of claim 7 wherein the electricmachine is coupled to a vehicle, the vehicle comprising: a vehicleframe; an axle coupled to the frame, wherein the stator is coupled tothe axle and the rotor is arranged circumferentially outside the stator;and a wheel rotatable on the axle, wherein at least one of the at leastone movable ring segment is moved away from the stator by centrifugalforce when the wheel is rotating at a speed greater than a thresholdspeed.
 14. A method to control a permanent magnet electric machinehaving a rotor and a stator, the method comprising: providing one of therotor or the stator with at least one movable iron segment; and movingat least one of the at least one iron segment away from the rotor orstator in order to weaken a magnetic field strength of the electricmachine.
 15. The method of claim 14 wherein at least one of the at leastone iron segment is moved away from the rotor or stator based on anoperating condition of the electric machine in order to achieve adesired magnetic field strength that allows operation of the electricmachine at a high efficiency condition.
 16. The method of claim 15wherein the operating condition comprises a requested power level whenthe electric machine is operating as a motor and wherein the requestedpower level is based on an operator demand of the electric machine. 17.The method of claim 15 wherein the operating condition comprises arequired voltage when the electric machine is operating as a generatorwherein the required voltage is based on a load coupled to thegenerator.
 18. The method of claim 17 wherein the load is a battery. 19.The method of claim 15 wherein the operating condition comprises arotational speed of the electric machine.
 20. The method of claim 15wherein the operating condition comprises a request to change between apropulsive power mode, where the electric machine is operating as themotor, and a regenerative mode where the electric machine is operatingas the generator.